The present invention is directed to an apparatus and method for presenting a modulated output signal at an output locus, such as a signal generator apparatus and a method for its operation. The present invention is especially directed to signal generator apparatuses that are used in connection with modulating an output signal according to at least one characteristic of an input signal. Examples of such modulation are frequency shift keying (FSK) modulation and on-off keying (OOK) modulation, but the present invention is not limited to use with those modulation technologies.
Signal generating circuits, sometimes referred to as frequency synthesizers, commonly operate with their entire functionality operating all the time. That is, synthesizing circuitry is integrally involved with calibrating circuitry, and both synthesizing and calibrating circuitry operate all the time. One result of such an arrangement is that excessive current is used in operating the synthesizer resulting in low efficiency and excessive heat generation.
Prior art synthesizers, for example, apply a transmit data signal (i.e., the signal that is the basis for modulating the synthesizer output signal for a transmitter) to a capacitor network that is employed to pull a crystal off-frequency. This design requires tight tolerances in selecting the crystal and in selecting the capacitors. Tighter tolerances generally translate to higher costs in manufacturing a product.
In contrast, the present invention uses a crystal (or other reference signal source) as a reference only. Moreover, the present invention employs digital signaling to establish frequency levels for effecting the required modulation in direct response to a modulating signal, such as a transmit data signal for a transmitter. The present invention permits operational separation of synthesizing circuitry and calibration circuitry. This permits calibration circuitry to be operated intermittently such as upon occurrence of a particular event, for example on the occasion of beginning a transmitting cycle. Alternatively, calibration circuitry of the present invention may be operated in response to a timer upon the elapsing of a predetermined time interval. Combinations of timed operation and occasion-instituted operation may also be employed with the present invention. For example, calibration may be carried out on commencement of transmitting operations, but may also be instituted according to a timed elapsing of an interval when long periods of transmission occur. Such timed calibration may be employed to guard against wandering of output frequencies over time by a transmitter.
Provision of an independently operated calibration function means that current is not being used by the calibration circuitry at all times during which the apparatus is operating. As a result less current is consumed by the apparatus, there is less heat build-up and the apparatus operates more efficiently.
Basing modulated output frequencies on digital signaling permits use of lesser tolerances in components in the apparatus of the present invention. Calibration at intervals, or on occasions or both at intervals and occasionally further contributes to accommodation of lesser tolerances in constructing a synthesizer using the present invention. Cost of manufacture may be thereby reduced without sacrificing reliability, operating efficiency or accuracy of operation.
There is a need for a frequency synthesizer apparatus that uses less current than prior art synthesizer circuits.
There is a need for a synthesizer circuit that can meet performance criteria using components having less stringent tolerances than are required for prior art synthesizer apparatuses.